Dozens of human embryos with three parents have been created by British scientists, ushering in an era of designer babies. The embryos – which effectively have two mothers and one father – have been genetically engineered to be free from incurable muscle, brain, heart and digestive illnesses, some of which kill within hours of being born.

The Newcastle University researchers say that within three years, it could allow women whose families are blighted by disease the chance of bringing a healthy child into the world.

But critics say the breakthrough is a step towards human cloning and erodes the sanctity of human life.

The research centres on mitochondria – powerhouses inside cells which turn food into energy to be used by the brain and body.

Each mitochondrion has its own DNA and is passed down from mother to child.

Serious defects in this DNA affect one in 6,500 babies and cause around 50 genetic diseases, some of which kill in infancy.

With no cure for the conditions, which include some forms of diabetes, blindness and heart problems, women carrying diseased mitochondria often face the heartbreaking choice of whether it would be kinder to remain childless.

The scientists have found a way of swapping the diseased DNA with healthy genetic material, creating embryos free of mitochondrial disease.

The ‘transplant’ technique, which is described in the journal Nature, involves using IVF techniques to fertilise an egg from a healthy donor.

When the resulting embryo is just a few hours old, the nuclear DNA, or genes, from the sperm and egg are removed, leaving the healthy mitochondria behind.

The would-be mother’s egg is then fertilised with her partner’s sperm and the nuclear DNA removed and put into the donor egg.

This creates an egg where the genetic material comes overwhelmingly from the prospective parents and the mitochondria are healthy.

If the method is successful, the disease should be eradicated from future generations of the family.

Professor Alison Murdoch, head of the Newcastle Fertility Centre, whose patients donated eggs for the study, said: ‘It would be hype to say we are going to get rid of mitochondrial disease but I think it’s realistic to say you could get rid of it in an individual family.’

Eighty embryos were created in the Newcastle labs, each effectively with three parents – two mothers and a father.

A fourth parent – the man whose sperm was used to fertilise the donor egg – was involved, but none of his DNA was passed on.

Some of the embryos lived for six days, before they were destroyed to comply with fertility laws, which also forbid such embryos from being implanted in a woman.

But updated fertility laws which came into effect last year leave the door open for the legislation to be amended quickly.

Lead researcher Professor Doug Turnbull said if this happened, the first babies could be born in as little as three years.

He said: ‘This is a very exciting development with immense potential to help families at risk from mitochondrial diseases.’

But Josephine Quintavalle, of campaign group Comment on Reproductive Ethics, said the technique was a step towards human cloning.

She added: ‘We know very little about the beginning of life and it is extraordinary how willing we are to break down one of the most obvious barriers, which is that it takes a sperm and an egg to create an embryo. We have got to find better ways to cure these diseases.’